1. Field of the Invention
The present invention relates to a limiter circuit suitable for use in a vehicle mounted receiver such as car radio or vehicle mounted CB transceiver, and more particularly to a limiter circuit for limiting pulsive noises superimposed on a modulation signal.
2. Description of the Prior Art
A vehicle mounted receiver such as an AM/FM radio receiver or CB transceiver is apt to be influenced by pulsive noises generated from an internal combustion engine of the vehicle.
Accordingly, the vehicle mounted receiver usually incorporates in a demodulator a limiter circuit for limiting the pulsive noises.
A limiter circuit as shown in FIG. 1 has been known as the limiter circuit for that purpose. In FIG. 1, there are connected between an anode of a detecting diode 11 of a detector circuit, that is, a detection signal input terminal T.sub.1 and a detection signal output terminal T.sub.2 a limiting diode 21, dividing resistors 22 and 23 for dividing a modulation signal (demodulation signal) which is developed at the input terminal T.sub.1 of the detection by the detecting diode 11 and a D.C. voltage to supply the divided signal and voltage to an anode of the limiting diode 21 and a filter circuit 30 which comprises resistors 31 and 32 and a large capacitance capacitor 33 for extracting only the D.C. voltage from the modulation signal and the D.C. voltage applied to an input terminal thereof to supply the D.C. voltage to a cathode of the limiting diode 21. During normal reception state, the limiting diode 21 is biased to be conductive, and it is cut off only when the pulsive noise is superimposed on the modulation signal and it exceeds an AM 100% modulation level so that the potential at the anode of the limiting diode 21 falls below the potential at the cathode thereof by the noise. In this manner, the noise is limited. One example of such prior art limiter circuit is disclosed in Japanese Patent Publication No. 15224/67 published Aug. 23, 1967 which was also assigned to the assignee of the present invention.
FIGS. 2A and 2B show waveforms for illustrating the operation of the limiter circuit. In FIGS. 2A and 2B, numeral 1 denotes a ground potential, 2 denotes a D.C. bias potential at the cathode of the limiting diode 21, 3 denotes a D.C. bias potential at the anode of the limiting diode, 4 denotes a modulation signal at AM carrier 100% modulation, 5 and 6 denote pulsive noises superimposed on the modulation signal 4, 4' denotes a modulation signal when AM carrier modulation does not reach 100%, and 5' and 6' denote pulsive noises superimposed on the modulation signal 4'.
In the limiter circuit shown in FIG. 1, a limiting level is determined by the A.C. voltage (AGC voltage) which depends on the level of the carrier. As a result, when the limiting level is set to the modulation signal amplitude at the AM 100% modulation as shown in FIG. 2A, the noise can be fully limited at the high modulation as shown in FIG. 2A but at the low modulation as shown in FIG. 2B the pulsive noise output is large relative to the modulation signal and the limiting effect is materially decreased.
If the limiting level is lowered in order to overcome the above difficulty, the modulation signal at the high modulation is limited and the waveform of the modulation signal is distorted. Accordingly, the limiting level has to have been compromised at an appropriate level.
Furthermore, as the demand for implementing electronic circuits by IC structures increases recently, it may also be desired to implement the circuit of FIG. 1 by an IC structure. In this case, the terminals T.sub.1, T.sub.2 and T.sub.3 must be provided as terminal pins of the IC. This is contrary to the object of the IC of permitting inclusion of as many functions as possible with a limited number of IC pins. In addition, relatively large capacitance capacitors designated by 13, 24 and 33 must be off-chip mounted.